IFR aircraft with PCT at 4000 landing IAD failed to adhere to speed requirements below Class B airspace; citing ATC failed to insure aircraft remained within the confines of protected airspace.

Narrative

We were west of IAD being vectored northeasterly; for landing at IAD. A descent to 4;000 FT was given by Approach. AML was being monitored on #2 DME; and being on a vector; #1 was set up for the ILS. When descent clearance was given; I believe we were a little more than 20 miles from AML. After monitoring the autopilot leveloff at 4;000 FT; I noticed we were just under 20 miles from AML (below Class B airspace). Began slowing aircraft to 200 KTS. We may have gone back out beyond 20 DME for a little while before being turned toward the final course; but did not increase speed. Class B airspace was designed to accommodate aircraft landing and departing the primary airport. It helps with safety conflicts and separation with aircraft using other airports. When landing at a primary airport; I would like to see it used without dropping any aircraft underneath its veil of protection. And if it has to happen; Approach should let their intent be known; so that speed can be decreased and vigilance increased.